Method of making cellulose acetate



June 12, 1962 A. MYTHOMSEN 3,038,894

METHOD 0F MAKING cELLULosE ACETATE Filed March 25, 195e J2, Lt Maf] Jute d'0 L. HJVENTOR- WMO United States Patent() 3 333,894 METHOD F MAKIG CELLULOSE ACETATE Alfred IVI. Thomsen, 265 Buckingham Way, Apt. 402, San Francisco, Calif. Filed Mar. 25, 1959, Ser. No. 801,885 3 Claims. (Cl. 260--227) The aim and object of my process is to take wood or other woody materials, meaning by that term vegetable material consisting largely of lignified cellulose, and so rearrange the cellulose and acetyl groups contained therein that the principal product shall be a high grade form of cellulose acetate.

'I'hat such a process is feasible is indicated by the composition of coniferous wood which will rarely give more than 32% of alpha-cellulose, after mercerization, while the acetic acid produced in a caustic fusion will result in almost half as much. Inasmuch as the composition of the commercial cellulose acetates vary from a low of 35% acetyl to a high of 41% acetyl, it follows that both alpha-cellulose and acetic acid in wood correspond roughly to the demands of the industry. It will only be necessary to supply a small amount of acetic acid from an outside source or else to segregate alittle of the pulp produced to produce a satisfactory balance of materials. In describing my process I shall follow the drawing and explain each step in detail so that the composite will represent a preferred version specifically pointing out what I consider as my invention.

Before commencing, however, I wish to call attention to the production of sodium phosphate as shown at the lower right portion of said drawing. This may seem out of place inasmuch as it is the result of an antiquated method of making acetic anhydride. Nevertheless, in its place it is a very efficient step and with the cheap phosphorus of today and the enormous market for sodium phosphates, it becomes of much economic importance. Link this with the fact that my recovery of acetic acid is entirely in the intermediate form of sodium acetate and the propriety of the step becomes immediately apparent.

Considering that such production of sodium phosphate is at least equal to one-half of the weight of the cellulose acetate produced, and through liberal use of the anhydride, in place of glacial acid, may become considerably more it becomes obvious that the source of such soda is of importance. In addition there is, of course, the unavoidable replacement soda, due to losses throughout, so I have indicated in the drawing that salt is fed to electrolytic cells as the source of both the soda and the chlorine called for in the process. This step has, of course, no place in my process, per se, and is introduced only as circumstantial information, hence, it is absent in the claims.

Commencing, therefore, with the digester at the upper right, I have shown wood treated with soda in the conventional way to produce pulp and a spent cooking liquor, separation being made in the blowpit just below. The pulp thus produced passes to the bleacher where it meets chlorine from the electrolytic cells and it is then mercerized in the step next below in an entirely conventional way. Hence, no description is required. Leaving the mercerizer I have indicated the pulp as alphacellulose, and the liquor as a solution of sodium hydroxide, rendered impure by the pulp impurities removed in said mercerization.

While I have called such mercerization conventionaL I mean that the operation may be so regarded. As a step in making alpha-cellulose it is anything but conventional. Hence, true alpha-cellulose is not used in making cellulose acetate, the cost of re-using the enormous excess of soda demanded by a 171/2 caustic being economically dfigd Patented .lune l2, i962 ice impossible. In current use is a cellulose so refined that it becomes acceptable, but not true alpha-cellulose. That is a product for the laboratory. Nevertheless, the superiority of a product which, after acetylation, dissolves in glacial acid without the formation of cloud is a thing desired.

I have solved the problem in presenting a use for this used caustic. Turning to the righthand again I have shown the spent cooking liquor evaporated in an evaporator and then commingled in the fusion kettle with said caustic solution. It is the aim to have such proportion of either substance that the mixture shall consist of three times as much caustic soda as there are organics present. Evaporation continues in the fusion kettle until the temperature of the mix reaches 230 C. It is then passed on to the leacher for further treatment. In the event that there is not enough caustic soda available from the mercerizer I have shown two other sources of re-cycled caustic all meeting in the fusion kettle.

In the leacherjust enough water is added to render the resident sodium acetate, made in the fusion from the acetyl groups in the wood, soluble, so that separation can be made between the solution of caustic and acetate, on the one hand, and undissolved material on the other. The latter material is a mixture of organics, sodium carbonate with some oxalate. I have shown the caustic solution re-cycled to the fusion kettle, the undissolved residue to the neutralizerf However, before the caustic solution reaches the fusion kettle it is concentrated and crystallized, yielding an abundant crop of sodium acetate crystals.

I have shown a centrifuge as the means for separating the crystals from the mother liquor. The latter is essentially a strong solution of caustic soda containing a little dissolved acetate. Being re-cycled to the fusion kettle it is immaterial what percentage is removed in the centrifuge, the balance being obtained on the next throughpass. The separated crystals are next passed to a drier where the water of crystallization is removed and the dried product next goes to the still in which P205 is added in sufficient amount to combine with all soda. At a gentle heat, below C. the reaction is complete, the acetic anhydride volatilizes and a residue of sodium phosphate remains.

The residue from the leacher will now be further processed. l have shown it going to a furnace, which may be a rotary kiln or a Herreshoif furnace or any other device which will agitate the charge while burning. It contains too little carbon to be smelted in the orthodox manner so the carbon is allowed to burn slowly with such added extra heat as may be needed. But before reaching the furnace use is made of the sodium carbonate it contains. At a later step in the process I obtain a weak solution of acetic acid which I commingle with this residue from the leaeher in a device called a neutralizer, which may be simply an agitated tank. I have shown this solution of sodium acetate, produced in said neutralizer, passed on to the evaporator-crystallizer before referred to and its acetate content is thus recovered as a component of the crystallized sodium acetate before described. The residue from the neutralizer is then sent to the furnace as before explained. The ashes from the furnace, consisting of sodium carbonate, is then treated in the conventional manner in the causticizer by adding water and lime. The caustic soda thus made is then re-cycled to any place where needed. I have indicated it as sent to the digester, to the fusion kettle, and to the mercerizer, respectively.

Having indicated the source of the acetic .anhydride I next return to the wet alpha-cellulose yielded by the mercerizer. In the Saturator, which is but a tank with a false, perforated bottom, this wet product is covered with acetic acid which is allowed to slowly displace the water, both within and between the fibers. A layer of wet material some 4 feet thick will require about 5 hours until acid of almost glacial strength flows from beneath the false bottom. This weak acid was used as previously described under the function of the neutralizer. But in this displacement of water by acetic acid much more than mere displacement has taken place. In fact, the alpha-cellulose has been converted into the mono-acetate without the need of any anhydride.

inasmuch as glacial acid, by itself, will only acetylate to an extent of 7% acetyl, while the mono-acetate contains 21% acetyl, it is obvious that the herein disclosed technique is of great value in acetylation. This monoacetate is next passed on to the acetylator, proper, where it meets the anhydride previously described. From this point on, full acetylation, solution in glacial acid, hydrolysis, and final precipitation is entirely conventional so no instructions are needed. The weaker acid, thus resulting, is then converted into glacial in the concentrator, any suitable solvent being used at the discretion of the operator, and recycled to where it may be needed. As before, being entirely conventional and with half a dozen systems in commercial use, any description would be superfluous. I have thus shown, in full, every step of which my process consists, those that are conventional as well as those which I have elaborated into a more useful status. Even if they were all conventional, the combination as expressed herein, constitutes a great advance over the present method of preparing the acetate.

As represented in the drawing my process is seen to consist of three parte. An entirely conventional Way of dividing a lignified cellulose into two parts, namely, pulp and spent liquor. A further refining of the pulp thus produced into an alpha-cellulose of such mechanical properties that it is convertable into the mono-acetate through glacial acid alone. Then, an alkali treatment of the evaporated spent liquor to liberate in the form of sodium acetate all the acetyl grouping present in the parent ligniiied cellulose, and the conversion of said sodium acetate into acetic anhydride by means of phosphorus pentoxide. Finally, the combination of said cellulose monoacetate with said anhydride, in a bath of glacial acid to produce the type of cellulose acetate desired.

From an economic standpoint, however, these so-called divisions require one another in a composite process as otherwise there is no place for the use of the side-products so liberally produced throughout. I have already called attention to the fact that the reason many of these reactions are not in daily use is because were they used they would prove economically unsound. But for the fact that it takes a very large excess of alkali to produce acetic acid in the form of sodium acetate there would be no place for mercerization, on one hand, and for the use of phosphorus pentoxide on the other hand. The composite constitutes a workable entity.

It will, therefore, be apparent why I have called my preferred version that which I have illustrated in my drawing and have now thoroughly explained. Any modification will be represented by some omission rather than by some addition and such omission will evidently represent an economic loss though the process will remain workable. Assuming the operator to be conversant with all that pertains to acetylation .and acetic acid chemistry, every step involved will appear clear and lucid.

Having thus fully described my process, I claim:

1. The manufacture of cellulose acetate from lignified cellulose which comprises; digesting said lignifed cellulose with a solution of caustic soda until the resident fibers have been substantially freed from lignin and other incrusting substances; separating said fibers from the resulting spent liquor; bleaching and mercerizing said pulp, the caustic soda solution resulting from said mercerization being commingled with the spent cooking liquor,

previously made, in an alkali fusion in which there shall be present approximately 3 parts of sodium hydroxide to 1 part of organic material the nishing temperature being approximately 230 C.; leaching the resultant product with enough water to dissolve the sodium acetate produced in said fusion, as well as the unused sodium hydroxide likewise present, and crystallizing out said sodium acetate; recycling the mother liquor from said crystallization back to the fusion step; commingling the sodium acetate with sufiicient P205 to combine with the contained sodium thus liberating acetic anhydride and volatilizing said anhydride 'for subsequent use in complete acetylation of the cellulose previously purified by mercerization; incinerating the residue from the leaching step to remove resident carbon and causticising the resultant sodium carbonate; recycling the solution of caustic soda thus produced to the mercerizing step, the fusion step, and the digesting step, respectively; displacing the water in the wet merccrized pulp with a re-cycled glacial acetic acid thus converting said cellulose into the monoacetate; commingling the weak acetic acid thus produced with the residue from the leaching step, prior to its incineration, and re-cycling the sodium acetate thus produced to the acetate crystallizing step; finally, commingling the mono-acetate with the acetic 'anhydrideY previously made to complete the acetylation of the cellulose.

2. The method of making cellulose acetate from lignified cellulose which comprises; digesting said lignified cellulose with a solution of caustic soda until the resident fibers are substantially freed from lignin and other incrusting substances; separating said fibers from vthe spent cooking liquor; bleaching and mercerizing said fibers to obtain a pulp consisting essentially of alpha-cellulose; displacing the water in the resultant wet pulp with a recycled glacial `acetic acid, thus converting said alphaceliuose substantially into cellulose mono-acetate; commingling the resultant mixture of glacial acid and monoacetate with sufficient acetic anhydride produced from the above mentioned spent cooking liquor to complete the acetylation and solution of the cellulose .acetate formed in the glacial acid, thus obtaining a clear solution.

3. The method of making acetic anhydride `from lignified cellulose which comprises; digesting said lignified cellulose with a solution of caustic soda until the resident fibers are substantially freed from lignin .and other incrusting substances and removing said fibers from the spent cooking liquor; commingling said spent cooking liquor with sufficient sodium hydroxide to yield a mixture containing approximately three parts of sodium hydroxide to one part of organic material and fusing the mixture at a finishing temperature of approximately 230 C.; leaching said fused product with sufiicient water to remove as a solution both the sodium hydroxide and sodium acetate resident therein, and converting the sodium resident in the leached residue by conventional means into caustic soda .and recycling same to the initial digestion step and to the alkali-fusion step vwhere previously indicated; removing sodium acetate from the solution obtained in said leaching by crystallization and re-cycling the mother liquor to the fusion step; commingling the resultant sodium acetate, after dehydration, with suliicient P205 to obtain acetic anhydride which volatilizes, leaving a resi` due of sodium phosphate.

References Cited in the file of this patent UNITED STATES PATENTS 1,668,946 Clarke et al. May 8, 1928 1,912,747 White et al. June 6, 1933 1,930,895 Haller Oct. 17, 1933 2,040,801 Billing May 12, 1936l 2,716,058 Rapson et al. Aug. 23, 1955 

1. THE MANUFACTURE OF CELLULOSE ACETATE FROM LIGNIFIED CELLULOSE WHICH COMPRISES; DIGESTING SAID LIGNIFIED CELLULOSE WITH A SOLUTION OF CAUSTIC SODA LUNTIL THE RESIDENT FIBERS HAVE BEEN SUBSTANTIALLY FREED FROM LIGNIN AND OTHER INCRUSTING SUBSTANCES, SEPARATING SAID FIBERS FROM THE RESULTING SPENT LIQUOR; BLEACHING AND MERCERIZING SAID PULP, THE CAUSTIC SODA SOLUTION RESULTING FROM SAID MERCERIZATION BEING COMMINGLED WITH THE SPENT COOKING LIQUOR, PREVIOUSLY MADE, IN AN ALKALI FUSION IN WHICH THERE SHALL BE PRESENT APPROXIMATELY 3 PARTS OF SODIUM HYDROXIDE TO 1 PART OF ORGANIC MATERIAL THE FINISHING TEMPERATURE BEING APPROXIMATELY 230*C., LEACHING THE RESULTANT PRODUCT WITH ENOUGH WATER TO DISSOLVE THE SODIUM ACETATE PRODUCED IN SAID FUSION, AS WELL AS THE UNUSED SODIUM HYDROXIDE LIKEWISE PRESENT, AND CRYSTALLIZING OUT SAID SODIUM ACETATE; RECYCLING THE MOTHER LIQUOR FROM SAID CRYSTALLIZATION BACK TO THE FUSION STEP; COMMINGLING THE SODIUM ACETATE WITH SUFFICIENT P2O5 TO COMBINE WITH THE CONTAINED SODIUM THUS LIBERATING ACETIC ANHYDRIDE AND VOLATILIZING SAID ANHYDRIDE FOR SUBSEQUENT USE IN COMPLETE ACETYLATION OF THE CELLULOSE PREVIOUSLY PURIFIED BY MERCERIZATION; INCINERATING THE RESIDUE FROM THE LEACHING STEP TO REMOVE RESIDENT CARBON AND CAUSTICISING THE RESULTANT SODIUM CARBONATE; RECYCLING THE SOLUTION OF CAUSTIC SODA THUS PRODUCED TO THE MERCERIZING STEP, THE FUSION STEP, AND THE DIGESTING STEP, RESPECTIVELY; DISPLACING THE WATER IN THE WET MERCERIZED PULP WITH A RE-CYCLED GLACIAL ACETIC ACID THUS CONVERTING SAID CELLULOSE INTO THE MONOACETATE; COMMINGLING THE WEAK ACETIC ACID THUS PRODUCED WITH THE RESIDUE FROM THE LEACHING STEP, PRIOR TO ITS INCINERATION, AND RE-CYCLING THE SODIUM ACETATE THUS PRODUCED TO THE ACETATE CRYSTALLIZING STEP; FINALLY, COMMINGLING THE MONO-ACETATE WITH THE ACETIC ANHYDRIDE PREVIOUSLY MADE TO COMPLETE THE ACETYLATION OF THE CELLULOSE. 